An electrophotographic image forming apparatus is roughly classified into a one-component developing apparatus using one-component toner and two-component developing apparatus using two-component developer including nonmagnetic toner and magnetic carrier, to form a toner image.
Since the one-component developing method is suitable for compactification, but not for high-speed development, a high-speed and long-life image forming apparatus often employs the two-component developing apparatus. In the developing apparatus using the two-component developer, while carrier itself in the two-component developer is not reduced because it is not consumed and remains inside the apparatus, toner is consumed and reduced by development. Hence, in order to stabilize quality of an image, the toner is suitably replenished so as to maintain a constant toner concentration of the two-component developer.
Although the toner concentration can be controlled using various methods, generally used is a toner concentration sensor so that toner replenishment is performed when exceeding a predetermined value. As the toner concentration sensor, for example, a magnetic permeability sensor utilizing that a magnetic permeability of two-component developer varies in accordance with a toner concentration thereof, is used.
Furthermore, in recent years, a system for detecting a toner concentration with other sensors, in addition to the toner concentration sensor, has been considered. For example, Japanese Laid-Open Patent Publication No. 2000-214672 discloses an image forming apparatus which is provided with toner concentration detecting means, image density detecting means for detecting an image density of a reference toner image formed on a surface of an image bearing member, print coverage calculating means for obtaining a print coverage of an image from input image information, and toner replenishment control means for detecting toner consumption based on calculation result by the print coverage calculating means, so as to control toner replenishment by toner replenishing means based on the detected toner consumption, detection result by the toner concentration detecting means, and detection result by the image density detecting means.
Furthermore, Japanese Laid-Open Patent Publication No. 2004-125829 discloses an image forming apparatus provided with rotational frequency counting means for counting rotational frequency of a developer carrier, image dot counting means for counting dot number of an image formed on the developer carrier, and control means for performing developing operation from the developer carrier to an image bearing member when the image dot number in a given rotational frequency is less than a predetermined threshold.
The toner concentration sensor sometimes takes time for circulating developer, and therefore can not detect a toner concentration instantly. Hence, until all circulation is finished (for example, about 10 printing sheets), toner replenishment is performed based on a value of a print coverage of a document (or dot count number) to realize more accurate correction of a toner concentration.
However, in a case where, the magnetic permeability sensor and the print coverage of the document (or the dot count number) are used as means for judging control of a toner concentration, there is a problem that, when documents having a low print coverage continue (that is, when toner consumption is small and toner is replenished little by little based on the print coverage of the document), a solid density gets lower. Note that, replenishing toner little by little is referred to also as “toner drip”.
The problem is considered to be caused by deterioration of toner, in other words, by that toner near carrier is not replaced and therefore receives too much stress. Against this, as described in the Japanese Laid-Open Patent Publication No. 2004-125829, a method for forcibly discharging toner is considered.
However, the image forming apparatus described in the Japanese Laid-Open Patent Publication No. 2004-125829 employs one-component developer and bulk change of the developer is considerably large at the time of forced discharge, thus taking time for toner discharge/replenishment, furthermore, without having the toner concentration sensor, the image forming apparatus is not suitable for high-speed and high-density printing.
Furthermore, since the image forming apparatus described in the Japanese Laid-Open Patent Publication No. 2004-214672 employs three kinds of sensors, it takes time to obtain detection result and control is also complicated.